|Page 5 - Free Fall: An Accelerating Experience
Technically, an object is in free fall when gravity and only gravity is allowed to act on it. In other words, it is what you and I would generally refer to as falling. However, a ball thrown into the air, even though its initial motion is upward, is also said to be in free fall because the only force acting on it is that of gravity. Likewise, satellites in orbit around a planet or star are also considered to be in free fall. For more on orbits see Large Scale Effects Pages 4, 5 and 6.
Now, if you are particularly on the ball today, you might have already come up with the argument that an object thrown or dropped in the air is not really in free fall because it is not just experiencing the force of gravity but air resistance as well. This is true. However, in most situations, air resistance has little effect on the motion of objects in free fall. And, because we are only trying to educate you on the effects of gravity and not those of air resistance, we are simply going to assume that it does not exist. Now that weve got that settled, lets continue.
To really understand free fall you must also understand acceleration. Most people think acceleration is simply when an object increases speed. After all, the gas pedal in a car or truck which makes the vehicle move faster is called the accelerator. By its most exact definition however, acceleration is any change in an object's speed or direction. If an object is at rest or is traveling in a straight line at a constant speed we say it is moving uniformly. But if it is speeding up (positive), slowing down (negative), or even changing direction (angular), it is undergoing a type of acceleration.
When an object is in free fall near the Earth, it is being accelerated towards the center of the Earth at a constant rate. This rate is approximately 9.8 meters per second squared. This means that after being in free fall near Earth for one second an object will be traveling at a speed of 9.8 meters per second. After two seconds its speed will have doubled to 19.6 meters per second and after three seconds it will have tripled to 29.4 meters per second and so on. However, this is only near the surface of the Earth. If you dropped an object while standing on the Moon where gravity's pull is much weaker the object would accelerate much more slowly.
For more on gravity and acceleration see What is Gravity? Page 6.